With the recent spread of portable electronic equipment such as notebook computers, video camcorders, and personal digital assistances, nonaqueous electrolyte secondary batteries having high voltage and high energy density have come to be used widely as a power source. From the concern for the environmental protection, electric-powered vehicles and hybrid-powered vehicles utilizing electric powder as a part of motive power have already been put to practical use.
Various additives for electrolyte have been proposed to provide nonaqueous secondary batteries with improved stability or electrical characteristics. Examples of such additives include 1,3-propane sultone (see patent document 1 below), vinyl ethylene carbonate (see patent document 2 below), vinylene carbonate (see patent document 3 below), 1,3-propane sultone or butane sultone (see patent document 4 below), vinylene carbonate (see patent document 5 below), and vinyl ethylene carbonate (see patent document 6 below). These compounds are considered to form a stable film called a solid electrolyte interface covering the surface of a negative electrode, which film is expected to prevent reductive decomposition of an electrolyte. Other examples include disiloxane having an unsaturated group (e.g., vinyl) (see patent document 7 below), fluorosilane having an alkenyl group (see patent document 8 below), alkylenebisfluorosilane (see patent document 9 below), and fluorosilane having an ether group bonded (see patent document 10 below). These compounds are considered to be adsorbed onto the surface of a positive electrode to protect the positive electrode thereby to prevent oxidative decomposition of an electrolyte. However, the additives of this type repeat adsorption to and desorption from the electrolyte surface in high temperatures, so that their protective effect is not sufficient in high temperature storage.
Patent document 11 below describes that 1,2-bis(difluoromethylsilyl)ethane is usable as an additive for lithium batteries. The document, nevertheless, gives no disclosure about test results of batteries using the compound nor disclosure about a compound producing a synergistic effect with 1,2-bis(difluoromethylsilyl)ethane.